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What I have gotten from scripture is that both Cain and Abel's occupations, for a lack of a better word, were indicative as well as symbolic of the type of lifestyles they lead. Abel was a sheep keeper; we have seen scripture in the New Testament where Jesus instructed people to tend to His sheep if they loved him, amd that is what Abel did, while Cain was a worker of the ground which was cursed by God in Genesis 3:17. This suggests that he was in complete rebellion of what God would have had him in do in the first place, he was a worker of evil, if put in simple terms. This I've also concluded because of God's words in 4:10 " The voice of your brother is crying to me from the ground." The very same ground Cain 'worked'
Cain simply offered God something evil and cursed, and was unacceptable.
In Genesis 4:4, 5 (NLT version) ” The LORD accepted Abel and his gift, but he did not accept Cain and his gift.
The bible verses tell us that Lord first accepted Abel and then his offering. He humbled himself before the Lord and with fullness of spirit he gave sacrifices to Lord.
But in Cain's case, his body was present to give offering but he didn't do with his mind and soul, and so God neither accepted Cain nor his offering.
It is like praising God with mouth but not with heart.